Chemical reactions, in particular, a variety of thermal and thermochemical processes, such as pyrolysis or cracking, are traditionally utilized in petroleum refineries and petrochemical plants. With demand for energy increasing worldwide on the background of unstable pricing and restrictive environmental policy requirements, the oil and gas industry faces a number of challenges, such as development of production technologies with enhanced energy efficiency and reduced environmental footprint. Those are also issues to consider for the development of one of the main petrochemical processes—large-scale production of lower (low molecular weight) olefins.
Low-molecular olefins, such as ethylene, propylene and butylenes, are primary components of petrochemical industry and serve as a basic building blocks in commercial production of plastics, polymers, elastomers, rubbers, foams, solvents, and chemical intermediates, as well as of fibers, including carbon fibers, and coatings. Existing technologies for production of lower olefins, which comprise pyrolysis of medium weight hydrocarbons, such as naphtha or gasoil and light hydrocarbons like pentanes, butanes, propane and ethane, down to lightweight substantially unsaturated polymerizable components, are commonly implemented in tubular furnaces. The latter imposes severe restrictions on pyrolysis processes: due to the fact that the operation is based on heat transfer, to maintain a satisfactory temperature distribution inside the reactor tubes remains a challenge.
On the whole, cracking of hydrocarbons is typically optimized by regulating at least a process temperature, residence time(s) and partial pressure of hydrocarbons. In conventional cracking furnaces controllability over the above mentioned kinetic parameters is restricted due to limitations imposed by the structures of the existing reactor solutions.
For example, in conventional tubular reactors heat energy is delivered to a reaction space through the reactor walls, whereby the reactor apparatus acts as a heat exchanger. Since heat transfer from the tube walls to the process fluid has its physical limits, in some instances raising the temperature such, as to achieve a desired reaction outcome, is impossible. Additionally, changing/controlling pressure in the reaction space of the conventional tubular (axial) reactors is challenging.
Thus, due to insufficient feedstock heating rate in tubular furnaces duration of pyrolysis process increases, which results in a situation, when formed at initial stages olefins reside in the reactor furnace for sufficiently long time to begin entering into secondary reactions thus considerably reducing the outcome of a target product. One of secondary products is coke, which causes heat transfer problems in tubes and fouling of the downstream equipment.
Additionally, in existing reactor solutions temperature gradients are very high. Thus, the temperature along the reactor walls is typically very high in comparison to its' center (as viewed along the entire reaction chamber). Since the fluid flow is faster in the center than in the areas adjacent to the reactor walls, large temperature gradient causes severe coking problems.
Furthermore, in existing reactor solutions residence times (times that the feedstock-containing process fluid spends in the reaction space) remain outside the scope of optimization.
Traditional technology does not offer reasonable solutions to the problems mentioned above due the fact that at a time being the conventional pyrolysis furnaces have already reached their technical limits, in terms of modifying heat transfer rates and/or adjusting the reaction temperature parameters and the reaction outcome, accordingly.
Traditional process for producing low-molecular weight hydrocarbons by thermal degradation thus encounters a major problem of lack of controllability over an entire process, therefrom a range of the secondary problems arises as follows: 1. poor performance factor for tubular furnace reactors; 2. loss of valuable feedstock material; 3. long reaction times; 4. high secondary reactions rates; 4. high energy consumption; 5. non-optimum (less than possible) product yield and selectivity.